The present invention relates to an organic ultra-thin film, particularly a thin film ranging in thickness from 5 nm to 100 nm, and a method for making the same.
Various techniques for forming and fixing an organic thin film on a substrate have already been known. Among them are a chemical absorption method (K. Ogawa et al., Langmuir, 6, 851 (1990)), the Langmuir-Blodgett method (hereafter referred to as the LB method), a casting method and a spin-coating method.
An ultra-thin film of angstrom level, that is, a monomolecular layer can be obtained by the chemical absorption method or the LB method. In those methods, the film thickness can be increased by forming one monomolecular layer upon another, and the thickness can be controlled with high accuracy by adjusting the repetition of that layer forming process.
The chemical absorption method is the most suitable for forming an ultra-thin film of angstrom level with a very high bonding strength to the substrate. However, this method involves a difficulty in forming a thin film ranging in thickness from 5 nm to 100 nm. A thick film can be obtained by a repetition of forming the monomolecular layer as mentioned above. However, in forming a new monomolecular layer on the surface of the originally formed monomolecular layer, it is necessary to modify the surface of the originally formed monomolecular layer with a highly reactive functional group. This process is very troublesome. Furthermore, since molecules used in the chemical absorption method are generally have a length of several nm in maximum, and one round of forming monomolecular layer can increase the film thickness by only several nm. This means that to produce a film not thinner than 10 nm, for example, the monomolecular layer formation and surface modification have to be repeated alternately tens of times. Such repetition take much time and labor.
On the other hand, the LB method is well established unlike the chemical absorption method with regard to the step of laminating monomolecular layers. But the obtained film is fixed to the substrate by an ionic bond, and one layer is bonded to another by the ionic bond or an intermolecular force. Therefore, the film produced by this method has a fatal shortcoming of a low bonding strength.
With the casting method or the spin-coating method, the film obtained has a relatively large thickness of micron level, and it is difficult to form a thin film ranging in thickness from 5 nm to 100 nm.
A film can also be formed by graft polymerization. In this method, the thickness of the film can be regulated, to a certain extent, by controlling a polymerization time, a monomer concentration and other factors. But this method has a problem that the substrate has to be given a special surface treatment—modification with a specific functional group of the surface of the substrate. Furthermore, no high-precision surface treatment technique has been established yet.
Also taken up for study recently is formation of a mixed film of inorganic and organic substances so-called hybrid polymer using a sol-gel method. The techniques of this type are classified roughly into two processes as follows. One uses an organic polymer with a metal alkoxide introduced into the side chain directly. The other involves mixing an organic polymer and the metal alkoxide, and causing them to react in formation of the film. Both of these processes use such metal alkoxides as those with Si, Al, Ti, Zr and Zn. With either of the processes, however, it is difficult to form the hybrid polymer film not thicker than 100 nm. Another problem is that fine particles of inorganic substances such as silica are liable to be formed in the sol or gel containing metal alkoxide, in general. Therefore, a thin film obtained tends to take a structure with organic polymers dotted among the inorganic fine particles. In other words, according to the sol-gel method, it is hard to produce a thin film having a structure which can be deemed as an organic polymer film.
It has also been practiced widely to coat the substrate with a fluorocarbon resin film. However, the fluorocarbon resin film sticks to the substrate only mechanically and is not fixed to the substrate by a strong bond like a chemical bond. Thus, the fluorocarbon resin film easily detaches from the substrate. Such detachment can be suppressed by increasing the thickness of the film. However, if the thickness is increased, cracks are liable to occur in the film since fluorocarbon resin is generally low in physical interlocking and interaction among the molecular chains, which means a weak cohesive force.
As indicated above, with any of the conventional methods, it is difficult to form an ultra-thin film ranging in thickness from 5 nm to 100 nm firmly fixed on the substrate.